1. Field of the Invention
This invention relates to means for dissipating the thermal energy imparted to the anode of a rotating anode X-ray generating tube.
2. Description of the Related Art
In X-ray generating tubes a stream of electrons emitted from a cathode and accelerated to high energy strikes an anode surface to release electromagnetic energy in the form of X-rays. In many applications, it is desirable to narrowly focus the stream of electrons onto a small area of the anode. In addition, it is often desirable to maximize the energy of the electron stream in order to produce a large amount of high energy X-rays. When electrons strike the anode surface only a small fraction of their energy is converted to X-rays. Much of the energy is, instead, released as heat thereby elevating the anode temperature. The buildup of thermal energy in the anode is a limiting factor in the power output, longevity, and efficiency of X-ray generating tubes. The need for continuous use, high power X-ray tubes has become even stronger with the advent of new types of medical equipment, such as computer assisted tomography ("CAT" scanners).
The use of a rotating anode disperses the energy of the electron stream over a large area, while maintaining a narrow focal spot. Rotating anode X-ray generating tubes are now common, and the construction and operation of such is widely reported.
However, even with the rotating anode design the buildup of thermal energy in the anode structure remains a problem. Since the anode structure operates in a vacuum, heat cannot be carried away from the anode surface by convection. Some heat can be conducted to the exterior of the tube through the bearing structure of the rotating anode. However, heat buildup in the bearing structure is a major cause of tube failure. Generally, it is desireable to thermally isolate the bearing, thereby minimizing heat loss by the mode of conduction.
One approach to increasing the thermal capacity of rotating anode X-ray tubes has been to increase the radius and volume of the anode disk, thereby increasing the mass of material capable of storing the thermal energy imparted by the electron beam. However, such designs do not increase the capacity of the anode to dispose of thermal energy. Under continuous use, such designs also result in heat build-up, again posing the same problem. Moreover, this approach has the added disadvantage of amplifying the mechanical motions of the anode as it rotates and increasing the difficulty of maintaining the mechanical tolerances of the anode structure. This approach raises the overall moment of inertia of the anode, thereby necessitating greater input of rotational energy.
Another approach has been to incorporate materials with high thermal capacity and emissivity into the anode structure; see U.S. Pat. Nos. Re 31,568 and Re 31,560 for examples of such anodes. The use of graphite in the anode structure is now fairly common for these qualities. While materials can be chosen which store and dissipate heat more effectively, heat dissipation remains a problem as power levels are increased. The improvement presented by this approach does not fully meet the needs of modern high power X-ray tubes.
Still another approach has been the design of liquid cooled rotating anode tubes. Examples of inventions of this type are set forth in U.S. Pat. No. 4,405,876. Nonetheless, this approach has not enjoyed widespread commercial success in medical applications due, primarily, to the complexity of the tube design and the expense of construction and maintenance of such tubes.
Accordingly, it is an object of this invention to provide new and improved means for dissipating thermal energy from a rotating anode structure of an x-ray generating tube.
Another object of this invention is to provide simple means for limiting the buildup of thermal energy on a rotating anode structure without substantially increasing the mass of the anode or resorting to complex systems of liquid cooling of the anode.
Yet another object of this invention is to substantially increase the surface area of the anode without substantially increasing the mass or overall size.
As will be seen from the following description, the effectiveness of the anode structure described herein can be further enhanced by the proper selection of materials.